The present invention relates to a vehicle-mounted radar apparatus.
A conventional vehicle-mounted radar apparatus, an apparatus as shown in FIG. 8 is known.
In the drawing, an antenna transmission/reception section 21 is comprised of an antenna unit 22, a coupler 23, a voltage controlled oscillator 24, a frequency conversion unit 25, and a gain control unit 26. Further, a signal processing section 27 is comprised of a modulated-signal control unit 28, a frequency analysis unit 29, and an arithmetic-operation control unit 30. Reference numeral 31 denotes a mechanical driving unit.
In the vehicle-mounted radar apparatus configured as described above, the modulated-signal control unit 28 supplies a modulation signal to the voltage controlled oscillator 24 to allow the voltage controlled oscillator 24 to generate relatively high frequency radio waves subjected to linear frequency modulation. Then, the relatively high frequency radio waves subjected to linear frequency modulation and outputted from the voltage controlled oscillator 24 are radiated to space from the antenna unit 22 via the coupler 23. Meanwhile, received radio waves from an object which reflects transmitted radio waves are received by the antenna unit 22 and are supplied to the frequency conversion unit 25.
In the frequency conversion unit 25, a part of the transmitted radio waves from the coupler 23 and the received radio waves from the antenna unit 22 are mixed, and a relatively low frequency signal is generated. An amount of transition of a frequency based on the time lag of radio waves corresponding to the distance to the object and an amount of transition of a Doppler frequency based on the moving velocity in a case where the object is moving are added to the frequency of the received radio waves. Accordingly, information such as the relative distance to the object and the relative velocity are multiplexed with the relatively low frequency signal generated by the frequency conversion unit 25. The power of this multiplexed signal is set by the gain control unit 25 in such a manner as to assume an appropriate magnitude for each scanning of the transmitted radio waves and the transmitted radio waves by the mechanical driving unit 31, and the arithmetic-operation control unit 30 computes the relative distance, the relative velocity, and the like with respect to the frequency data from the frequency analysis unit 29.
The above-described radar apparatus is used for a following-distance warning apparatus which informs the driver of a danger by issuing a warning when a distance with a vehicle ahead has become shorter than a safe following distance and the danger of a collision has become heightened, or for a following-distance controlling apparatus for effecting follow-up traveling by keeping a safe following distance with a vehicle ahead.
Further, a technique for improving the distance resolution by changing a modulation period and a technique for facilitating pairing when an identical target is determined are disclosed in publications which are cited below as conventional techniques.
Namely, Unexamined Japanese Patent Publication 8-136647 discloses a technique in which the normalized value of (distance value/beat frequency) is made small by making a modulation period short, so as to improve the distance resolution.
In addition, Unexamined Japanese Patent Publication 8-189965 discloses a technique in which a modulation period is made long during high-speed traveling to widen the range of a detection distance, and the modulation period is made short during close-distance detection to restrict the detection range to the close distance, thereby improving the distance resolution.
Further, Unexamined Japanese Patent Publication 8-211145 discloses a technique in which if the modulation period is made large, the velocity resolution declines, but the difference in amplitude between reflected signals from a target which are obtained during a rise and a fall in frequency modulation becomes small, so that when an identical target is determined, pairing is facilitated by combining the same amplitudes.
Next, referring to FIG. 9, a description will be given of a method of determining a straight line-of-sight angle and a straight line-of-sight distance from a subject vehicle to a preceding vehicle traveling on the same lane as that of the subject vehicle when the subject vehicle is traveling on a monotonous curved road in a general road environment. FIG. 9 is an explanatory diagram illustrating one example for computing the straight line-of-sight angle and the straight line-of-sight distance.
In FIG. 9, the meanings of the respective symbols are as follows.
______________________________________ .theta. [unit: .degree.]: straight line-of-sight angle from the subject vehicle to the preceding vehicle traveling on the subject- vehicle lane R [unit: m]: straight line-of-sight distance from the subject vehicle to the preceding vehicle traveling on the subject- vehicle lane r [unit: m]: radius of the curve of a highway (defined at the center of a lane) W [unit: m]: lane width (in Japan, W = 3.5 m) t [unit: m]: distance of deviation from the center of the lane during traveling of the subject vehicle, and it is assumed that the subject vehicle travels within .+-.t [unit: m] from the center of the lane s [unit: m]: distance of deviation from the center of the lane during traveling of the preceding vehicle, and it is assumed that the preceding vehicle travels within .+-.s [unit: m] from the center of the lane ______________________________________
The following formulae can be obtained by geometrically analyzing FIG. 9. ##EQU1##
If the straight line-of-sight angle .theta. and the straight line-of-sight distance R with respect to a number of specific radii r of the curve (defined at the center of the lane) of a monotonous curved road are determined in accordance with Formulae (1) to (3), we have
Example 1. in the case of r=319 [unit: m] EQU .theta.(319, .+-.0).congruent.6[unit: .degree.], R(319, .+-.0, .+-.0).congruent.67[unit: m] (4)
Example 2. in the case of r=460 [unit: m] EQU .theta.(460, .+-.0).congruent.5[unit: .degree.], R(460, .+-.0, .+-.0).congruent.80[unit: m] (5)
Example 3. in the case of r=718[unit: m] EQU .theta.(718, .+-.0).congruent.4[unit: .degree.], R(718, .+-.0, .+-.0).congruent.100[unit: m] (5)
Example 4. in the case of r=1277[unit: m] EQU .theta.(1277, .+-.0).congruent.3[unit: .degree.], R(1277, .+-.0, .+-.0).congruent.134[unit: m] (7)
Example 5. in the case of r=2872[unit: m] EQU .theta.(2872, .+-.0).congruent.2[unit: .degree.], R(2872, .+-.0, .+-.0).congruent.200[unit: m] (8)
In a general road environment, it is considered that 160 m or thereabouts is required as the distance for detecting the preceding vehicle traveling on the subject vehicle lane from the subject vehicle.
However, in the case of monotonous curved roads, there are cases where the straight line-of-sight distance from the subject vehicle to the preceding vehicle traveling on the subject-vehicle lane is substantially shorter than 160 m, as shown in the results of Formulae (4) to (8) above.
Since the conventional vehicle-mounted radar apparatus is configured as described above, with respect to the direction of the straight line-of-sight angle from the subject vehicle to the preceding vehicle traveling on the subject-vehicle lane on a monotonous curved road, the detection distance becomes longer than is necessary, so that the subject vehicle receives radio waves received from a multiplicity of unnecessary objects such as vehicles.